Definitely grey. It's a neutral, not girly, is perfect with chrome accessories (faucet, door knob, etc) and makes the whites in the room look nice and crisp.
Go get yourself a nice grey bathroom carpet...they are usually that perfect grey...select your paint with carpet in hand.
OR
There are many types of grey (brownish, greenish, blue...) Get a bunch of paint samples and put them on the floor... you will immediately know what works with your lighting.
I personally would go with a dark grey because there is so little wall surface.

This is a nice safe one
http://houzz.com/photos/1289901

Find a colorful piece is art for the wall to set your color palette. Something that reflects the boys interests. It may even be a framed poster. Make it fun.

This is a nice one if you decide to go dark
http://houzz.com/photos/1241549

I like the greys and have decided on the mindful gray with a gray chevron shower curtain. I need a color that pops and won't clash with the floor.

The black countertop is a great foundation for this bathroom. Black says "bring it", especially in a teens bathroom. Go with bright red or a clear pure turquoise to make a bold statement with rug, shower curtain and towels. Depending on how large the bathroom is-- you could possibly do a bold accent wall too. The other walls, I would paint white. I wouldn't try to soften or neutralize this room.

This is my teen kid's bathroom-/they are scuba divers!